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Powder Coating over Galvanized Metals

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  • Powder Coating over Galvanized Metals

    Good Morning Gang,
    Here's some info that will help you with Powder Coating over Galvanized Steel

    Challenges with Powder Coating over Galvanized Metals:

    Hot dip galvanized surfaces have been proven to be difficult to powder coat since powder coating was first developed in the 1960’s. Industrial Galvanizers began research on this subject back in 1986.
    There are three main challenge points associated with powder coating on hot dip galvanized steel. The following will help you with this challenge.

    1. Solvent Pop in the film coating

    Out gassing is caused by the release of small gas bubbles into the powder film coating during the curing cycle. These bubbles form small craters on the surface and are unsightly. This called Solvent Pop. They also produce weaknesses in the film coating that reduce its long term durability, particularly in (marine) environments.

    The main reason for out gassing appears to be that the polyester resin particles coming in contact with the galvanized steel surface do not bond at the same time as those on the surface of the polyester powder film due to the mass of the galvanized steel.
    Specially formulated resins with 'degassing' agents such as OGF Primers have been developed to alleviate this challenge by delaying the onset of fusion of the powder.

    That means you get a longer flow out time before gelling with the OGF Primers. Pre-heating or Out Gassing the work prior to powder coating allows heavier or, larger mass, hot dip galvanized sections to be powder coated. Out Gassing deals with the problem of solvent popping and pin holing when used in conjunction with 'degassing' grades of polyester powder.

    Hot dip galvanized parts tend to have heavier section thicknesses than other steel items. Typically sheet steel parts that are powder coated. These items naturally take longer to reach oven temperature because of their greater mass.



    2. Poor Adhesion

    The final stage of the hot dip galvanizing process involves water quenching the part. This is frequently done in a weak sodium dichromate solution. This process cools the part down so it can be handled and passivizes the surface of the galvanized part to prevent premature oxidation.
    The presence of a passivating film on the surface of the galvanized part will impede the zinc or iron phosphate pretreatment. In many cases, Galvanizing parts will render these pre-treatments useless.
    If you are planning to use Iron Phosphate on your Galvanized parts do not quench the parts after the galvanizing process. The Zinc will stay in a reactive state and accept the Iron Phosphate Etching.

    It is paramount that after the part is hot dipped that the surface is kept dry and clean. The quenching step is left out so there will not be a finalized oxidation proofing on the surface until you etch with Iron Phosphate.
    If oxidation appears on your part and you apply powder film you will see oxidation come through the powder film after some time.

    3. Incomplete curing of the polyester powder resins

    Polyester powders are thermosetting resins that cross-link into their final form by baking at temperatures typically around 350 F to 400 F, for 10 to 30 minutes depending on the powders chemical formulation.

    Hot dip galvanized items sometimes have heavier section thicknesses. It is necessary to ensure sufficient baking time is allowed to meet the curing specifications in the part metal temperature of the curing schedule of the particular powder formulation being used.

    Pre-heating the larger mass parts will shorten the curing time. When you preheat parts you increase the transmission efficiency of the electrostatic powder coating system by nearly 30%. Be careful not to over load powder onto the parts. It will quickly stick like glue.




    Powder Coating Over Hot Dip Galvanizing

    The following process is recommended:

    1. Hot dip galvanizes the part then do not water or chromate quench leaving it open for your Iron Phosphate etching.

    2. Wear Latex Gloves and remove all surface defects. Use denatured alcohol and micro fiber towels.

    3. Powder coat within 12 hours or less after galvanizing. Do not get surfaces wet or dirty. Do not leave the parts outside.

    4. Keep the surface clean and dry. Do not transport uncovered loads. Truck diesel exhaust fumes will contaminate the metal surface.

    5. If surface contamination and/or oxidization has occurred clean the surface with denatured alcohol and micro fiber towels.

    6. Use iron phosphate etching for the best adhesion. Surface must be perfectly clean! Iron phosphate has very little detergent action so it will not remove oil or soil. Do all cleaning before the Iron Phosphate Etching.

    7. Pre-heat larger mass parts prior to powder application to shorten overall curing time.

    8. Use OGF Primers and polyester powders only.

    9. Check for correct film curing by solvent testing. Use MEK and Q~tips. Adjust pre-heat temperatures and line speeds to ensure the powder fully cures.

    Check out this Picture From Tony Bachelor. He attended our 4~Day Class a short while back and is moving forward very quickly! These are Galvanized Tin Roof parts he's been using as samples! NICELY DONE TONY!
    Attached Files
    Last edited by PowderGuru; 05-01-2014, 02:07 PM.

  • #2
    I now have an older galvanized chevy truck rear bumper to do. So i am about finished blasting it and then phosphate? or ogf primer? okay to use "wheel silver" ? then a clear coat? Do i need to outgas it too? thanks

    Comment


    • #3
      Hi rider1234,

      Well it's a bumper so I guess I can assume that is was made from cold rolled steel and stamped into shape. With this in mind you don't have to worry much about out gassing.

      If you want to provide your customer with the Iron Phosphate coating that will be up to you. Since it is going onto a Chevy you'll have to give every it bit of help you can... It's a Chevy, lol. Just kidding.


      OGF Primer is always a good idea for automotive parts. You get an Epoxy Hybrid base coat designed for corrosion protection a couple of extra mill of paint to boot.

      The wheel silver will benefit from the OGF base coat. After that apply the Clear.

      Now here's a trick you can use for inter coat adhesion:

      When curing the second coat use 30 minutes more on the time when shooting cold and 20 minutes more when shooting hot. when applying the 3rd coat add 30 to 50 minutes.

      Reason:
      The film that is on the part you coating over acts as an electrical and heat insulator and your oven must over come this to reach PMT.


      Do not exceed the powders cure temp. The part will be in a heat pocket for quite a while so be careful. The resins and pigments can burn. The first few times you do this you will start to see small changes in how the glosses look and respond to MEK Rub Tests.
      Last edited by PowderGuru; 05-01-2014, 02:33 PM.

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